The present invention relates to a rotary head drum unit and a magnetic tape drive, and more particularly to a technique for improving a contact condition between a magnetic head and a magnetic tape.
A rotary head drum unit is used to record a signal to a magnetic tape such as a tape-like magnetic recording medium or reproduce a signal recorded on the magnetic tape. Such a rotary head drum unit is mounted in a video tape recorder, for example, and this unit is composed of a fixed drum and a rotating drum. The rotating drum is provided with a magnetic head, so that when the rotating drum is rotated in one direction, the magnetic head comes into sliding contact with the magnetic tape to thereby record or reproduce a signal.
Referring to FIG. 22, there is shown a rotary head drum unit a in the related art. The rotary head drum unit a has a fixed drum b fixed to a chassis (not shown) and a rotating drum c having substantially the same outer diameter as that of the fixed drum b.
The rotating drum c is fixed to a rotating shaft (not shown) that is rotatable relative to the fixed drum b. The fixed drum b and the rotating drum c are axially opposed to each other with a given gap defined therebetween. The rotating shaft is rotated by a motor (not shown).
A plurality of recesses (which will be hereinafter referred to as “head mounting holes”) d are formed along the lower circumferential edge of the rotating drum c so as to be spaced apart from each other at given intervals in the circumferential direction. A magnetic head e is positioned in each head mounting hole d.
As shown in FIGS. 22 and 24, each magnetic head e projects from the outer circumferential surface of the rotating drum c.
The amount of projection of each magnetic head e from the outer circumferential surface of the rotating drum c is set larger than the flying height of a magnetic tape h above a drum surface (e.g., the cylindrical surface of the rotary head drum unit a), thereby obtaining a contact pressure of the magnetic tape h to a tape sliding surface i of the magnetic head e as will be hereinafter described.
As shown in FIGS. 23 and 24, each magnetic head e is mounted on a head substrate f, and each head substrate f is fixed to the rotating drum c at an arbitrary position in such a manner that each magnetic head e projects from the corresponding head mounting hole d.
The outer circumferential surface of the fixed drum b is formed with a lead guide portion g extending substantially helically for guiding the lower edge of the magnetic tape h helically wrapped around the rotary head drum unit a during running of the magnetic tape h.
When tape loading is carried out, the rotary head drum unit a is rotated and the magnetic tape h wrapped a given angle around the rotary head drum unit a is run in a given direction (see FIG. 22).
In the rotary head drum unit a, the rotating drum c is rotated in the condition where the magnetic tape h is running along the lead guide portion g, thereby making each magnetic head e scan the running magnetic tape h in a direction inclined a given angle with respect to the longitudinal direction of the magnetic tape h. Accordingly, the rotary head drum unit a forms a recording track extending in the direction inclined the given angle with respect to the longitudinal direction of the magnetic tape h.
When the rotating drum c is rotated, air is introduced between the drum surface of the rotary head drum unit a and the magnetic tape h, so that the magnetic tape h runs at a given flying height above the drum surface of the rotary head drum unit a (see FIG. 24).
This is due to the fact that an air layer is formed between the drum surface of the rotary head drum unit a during rotation and the magnetic tape h during running, and that a pressure difference between this air layer and the atmospheric air existing on the opposite side of this air layer with respect to the magnetic tape h is held constant in relation to a tape tension or the like of the magnetic tape h, thereby obtaining a given flying height of the magnetic tape h above the drum surface of the rotary head drum unit a during running of the magnetic tape h.
For example, in the case of a magnetic tape drive system having such specifications that the width of the magnetic tape h is 8 mm, the diameter of the rotating drum c is 40 mm, and the running speed of the magnetic tape h relative to each magnetic head e is 10 m/sec, it is said that the flying height of the magnetic tape h above the drum surface is usually 10 μm or more.
The amount of projection of each magnetic head e from the drum surface is set larger than the flying height of the magnetic tape h above the drum surface. Accordingly, when the magnetic tape h comes into sliding contact with each magnetic head e projecting from the drum surface of the rotary head drum unit a, the magnetic tape h forms a tent-shape at each magnetic head e according to the amount of projection of each magnetic head e (see FIGS. 22 and 24).
That is, the magnetic tape h is urged by each magnetic head e to form a ridge along the tape sliding surface i of each magnetic head e. In this condition where the tent-shape is formed in the magnetic tape h, a signal is recorded or reproduced.
The formation of the tent-shape in the magnetic tape h means that a portion of the magnetic tape h corresponding to the tape sliding surface i of each magnetic head e is curved. Accordingly, there is a possibility that the magnetic tape h may be partially separated from a central portion of the tape sliding surface i of each magnetic head e.
To cope with this, the tape sliding surface i of each magnetic head e is sometimes formed into a curved surface so as to stabilize the contact condition of the magnetic tape h to the tape sliding surface i, i.e., to uniform the contact pressure therebetween.
However, the tape sliding surface i of each magnetic head e has a size of 1 mm×100 μm, for example, so that it is not necessarily easy to form the tape sliding surface i into a curved surface.
Further, the tape sliding surface i may be formed into a curved surface only by predicting the curvature of the tent-shaped sliding portion of the magnetic tape h. However, the curvature of the tape sliding surface i does not always become equal to the curvature of the tent-shaped sliding portion of the magnetic tape h.
In particular, the tent-shape of the magnetic tape h depends greatly on the stiffness of the magnetic tape h. FIG. 25 shows a contact condition of a magnetic tape j having a high stiffness (which will be hereinafter referred to as “high-stiffness tape”) and of a magnetic tape k having a low stiffness (which will be hereinafter referred to as “low-stiffness tape”) with respect to the tape sliding surface i. In the case that the shape of the tape sliding surface i is designed to accommodate the high-stiffness tape j, the low-stiffness tape k may be separated from a central portion of the tape sliding surface i. Conversely, in the case that the shape of the tape sliding surface i is designed to accommodate the low-stiffness tape k, the high-stiffness tape j may come into contact with only a central portion of the tape sliding surface i.
The low-stiffness tape k is so curved as to follow the shape of the projecting magnetic head e more than the high-stiffness tape j, so that the slope of the tent-shape is steeper and the curvature of a top portion of the tent-shape (a portion facing the tape sliding surface i) is therefore larger. Accordingly, in the case that the shape of the tape sliding surface i is designed to accommodate the high-stiffness tape j, there arises a problem that the low-stiffness tape k does not come into contact with the tape sliding surface i (see FIG. 25), and the contact of the magnetic tape h and each magnetic head e becomes non-uniform, causing a reduction and variations in recording or reproduction output.
For the above reasons, the contact (contact condition) between each magnetic head e and the magnetic tape h in the initial stage of use of the rotary head drum unit a is not stable, so that “poor contact” may frequently occur. In many cases, the contact condition of the magnetic tape h to each magnetic head e becomes better when the magnetic head is worn after use of the rotary head drum unit a.
Further, the tape sliding surface of each magnetic head may be polished to match the tent-shape by using a polishing tape during the manufacture of the rotary head drum unit. However, this method causes an increase in manufacturing time, and the stiffness of the polishing tape is not always equal to the stiffness of the actual magnetic tape. The stiffness of the actual magnetic tape differs according to the kind of tape, the manufacturer of the tape, etc. Therefore, the above method using the polishing tape is not an effective means for solving the problem.
Other means for solving the problem are described in Japanese Patent Laid-open No. Hei 11-273021 and No. 2001-297417. The solution described in the former publication is forming a recess in the vicinity of the gap on a magnetic head. The solution described in the latter publication is forming a recess in the vicinity of a magnetic head on a drum surface. By forming such a recess, the volume of the recess is made larger than that of another portion, thereby obtaining negative pressure in the recess.
Accordingly, the leading and trailing edges of the tent-shape of the magnetic tape h are attracted into the recess by negative pressure, thereby making the tent-shape uniform to suppress variations in the contact pressure of the magnetic tape with respect to the magnetic head irrespective of a difference in stiffness between magnetic tapes.
However, in each of the above publications, the magnetic head projects from the drum surface by an amount larger than the flying height of the magnetic tape to form the tent-shape of the magnetic tape. This configuration is basically similar to that shown in FIGS. 22 to 24.
Where the recess is formed on the drum surface in the vicinity of the magnetic head or on the tape sliding surface of the magnetic head, the leading and trailing edges of the tent-shape are attracted into the recess by negative pressure, resulting in positive contact, between and increased contact pressure between the magnetic tape and the tape sliding surface of the magnetic head. As a result, there is a possibility of increase in wear or damage of the magnetic head and the magnetic tape.
Further, in association with a recent demand for high-capacity recording, a magnetic tape is elongated and the thickness thereof is therefore reduced. A thinner magnetic tape is lower in stiffness than a conventional magnetic tape. A low-stiffness magnetic tape is more susceptible to physical variations than a high-stiffness magnetic tape. That is, the contact pressure between the low-stiffness magnetic tape and the magnetic head is easily varied to result in acceleration of a reduction and variations in recording or reproduction output.
Further, in response to high-speed rotation of the rotary head drum unit a required for high-speed transfer of data and broadening of the band of recording and reproduction signals, it is required to suppress variations in the contact condition between the magnetic tape and the tape sliding surface of the magnetic head, and the conditions for obtaining a good contact condition are narrowed.
While the rotary head drum unit a in the related art mentioned above is of an upper-drum rotating type, the above problems may also occur with lower-drum rotating type and intermediate-drum rotating type rotary head drum units.